_node is not a private class member.

Extend and unify documentation, fix function's argument name.

Don't confuse doxygen.

The vector is filled with pointers to base nodes.

The existing clone function is a direct copy, but
setting different nodes is required.

Since boost-1.61 (and also not in the std::optional)
optionals holding references do have some restrictions
and are difficult to use. In this special case
optional<PropertyVector&> is equivalent semantically
to a PropertyVector*. The pointer indicates that it is
not owning (contrary to usage of unique_ptr).

Thanks to Christoph pointing this issue out.

Allow modification of the component of the specified tuple.